The present invention relates generally to the field of radar systems and, more particularly, to radar systems which are mounted on non-stable platforms such as those installed on ships, aircraft or land vehicles such as tanks. Current two-dimensional search radar systems installed on such craft and vehicles suffer from azimuth errors and PPI (plan position indicator) display ambiguities whenever the craft on which the radar system is installed rolls or pitches.
Ship's heading as used herein means the instantaneous direction of the ship's longitudinal axis in the horizontal plane when pitch and roll are both zero. The heading, together with the vertical axis through the mass-center of the ship, and the third axis that is mutually perpendicular to these two, form the reference frame in which the pitch and roll angles are defined. The true bearing (azimuth) is defined as the clockwise angle in the horizontal plane, relative to the ship's heading, of the vertical plane containing the line-of-sight to the target. The radar azimuth signal .alpha. is the clockwise instantaneous angle in the deck plane of the ship from the ship's forward longitudinal axis to the slant plane perpendicular to the deck plane which contains the line-of-sight to the target.
The pitch angle, .theta., is the angle of rotation, with regard to a ship, resulting when the forward end of the ship rises about the axis of the deck plane of the ship, perpendicular to the longitudinal axis. The roll angle, .phi. of a ship, for instance, is the angle of rotation of the ship about the longidutinal axis of the deck plane of the ship. .theta. is defined to be positive when the forward end of the ship rises about the axis of the deck plane and .phi. is defined to be positive when the starboard side of the ship tilts down about the longitudinal axis of the deck plane.
Whenever such pitch or roll occurs, the radar system installed on the vehicle or craft suffers from the aforementioned azimuth error and PPI ambiguity. Thus, the angle output of the radar of the system, at the time its beam illuminates a target, is not necessarily equal to the true azimuth of the target in the horizontal plane. Consequently, one stationary surface target can cause a sequence of disconnected blips on a PPI display as the ship rolls, on successive sweeps of a target by the rotating beam of the radar system.
A further error may occur if the target is an airborne target. Many search radars in the navy do not measure the elevation of the target. They simply sweep a beam which has a wide fan shape in a general up-down direction and, if this beam illuminates a target at any elevation whatsoever, a blip is recorded on the PPI display. The errors associated with the azimuth of airborne targets are typically larger than those associated with surface targets. There are hundreds of radar systems in use in the navy at this time having the aforementioned limitations.